In folklore, brownies are said to perform chores at night while the residents sleep.
Brownies are household spirits of Scottish and northern English tradition. Stories describe them quietly cleaning homes, tending animals, threshing grain, churning butter, or completing other farm tasks after the human occupants have gone to bed. Their nighttime activity helps explain why the work is discovered rather than witnessed.
Families were expected to leave an offering, commonly milk, cream, porridge, or a small cake, near the hearth. In return, the brownie helped the household prosper. The spirit could also become mischievous or punitive, especially when servants were lazy, when the household was ungrateful, or when someone tried to watch it at work.
Night is the canonical answer, not sunset or dawn. Folklore sometimes varies in its details, but the defining pattern is nocturnal labor carried out while the household is asleep. The same tradition also says brownies may leave if insulted or given clothing.